This invention relates generally to aircraft braking systems, and more particularly concerns a method for maintaining optimal braking and skid protection for a two-wheeled vehicle wheel with a wheel speed sensor failure.
Automatic braking systems commonly have been provided on commercial, military, and large turbine aircraft to aid the deceleration of the aircraft upon landing. Modern aircraft braking systems typically optimize braking efficiency by adapting to runway conditions and other factors affecting braking to maximize deceleration, corresponding to the level of brake pressure selected by the pilot. Antiskid systems for military, small business jets and general aviation airplanes have traditionally utilized antiskid control systems that sense wheel speed from left and right wheel speed sensors.
Individual wheel antiskid control systems typically use separate control channels for each wheel and individual wheel antiskid valves. Peak efficiency is maintained on both wheels, so that the shortest stopping distance is achieved. However, in the event of a failed wheel speed sensor, there exists the possibility that brake pressure reduction on one wheel due to antiskid activity will cause a shift in airplane direction during a temporary brake pressure imbalance.
There were previously two options in the event of a failed wheel speed sensor. One option was to inhibit braking on that wheel. This is not practical on some aircraft, especially with only two braked wheels, because of the loss of directional control and the increase in stopping distance. The second option was to pulse the brake pressure on and off up to the brake pressure command from the pilot's brake pedals. This method prevents blown tires and allows directional control, but causes significant loads on the landing gear structure due to the occurrence of periodic skid events caused by brake pressure application. These loads can be high enough to cause excessive wear and possibly failure of the landing gear.
It would be desirable to provide pulsed brake pressure to a wheel with a speed sensor failure, to periodically release brake pressure on the wheel, allowing the wheel to spin up, in order to protect against tire failure due to lockup. Particularly for a two-wheeled vehicle with a wheel affected by a speed sensor failure, it would also be desirable to apply a fraction of the skid pressure from another wheel with a functioning wheel speed sensor to the affected wheel to prevent the brake pressure on the affected wheel from being large enough to skid, in most conditions. It would also not be desirable to inhibit braking on a wheel with a failed speed sensor, because maintaining braking on the affected wheel allows the pilot to maintain directional control using differential braking. For a two-wheeled vehicle with a wheel affected by a wheel speed sensor failure, it would also be desirable to limit the brake pressure on the affected wheel to a value below the skid brake pressure on the opposite wheel, in order to greatly improve stability margins on the landing gear. The present invention meets these and other needs.